`RDRSv2.1` processing script could be more efficient

[kasra-keshavarz]

Reported by: Sujata Budhathoki @sujata91

The process time for RDRSv2.1 is inefficient as it takes nearly 3 hours for any domain. This could be much more efficient and be done in a few minutes for any spatial/temporal domain of interest.

Solutions that come to mind:

1. first subsetting needs to be done and then the modifications on the lon value take place,
2. spatial subsetting to be done using ncks index values, rather than cdo's sellonlatbox method.

[kasra-keshavarz]

Partially resolved with #47